Rate responsive compensation change circuit



Aug. 3, 1965 c. B. BRAHM ETAL RATE RESPONSIVE COMPENSATION CHANGE CIRCUIT med May s1, 196s 3 Sheets-Sheet 1 Aug. 3, 1965 C. B. BRAHM Em. 3,199,013

RATE RESPONSIVE COMPENSATION CHANGE CIRCUIT Filed May 3l, 1965 3 Sheets-Sheet 2 ll. wp.

@y 9mm a1 BM ,afro/e/f/ Aug. 3, 1965 C. B. BRAHM ETL RATE RESPONSIVE COMPENSATION CHANGE CIRCUIT Filed May 31, 1963 5 Sheets-Sheet 5 United States Patent O 3,199,013 RATE RESPNSVE COMPENSATION CHANGE CIRCUIT Charles B. Brahim, Ellington, and John E. Games,

Granby, Conn., assignors to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Filed May 31, 1963, Ser. No. 284,707 Claims. (Cl. S18-489) This invention relates to a stability augmentation system for aircraft, and particularly to a system which will stabilize a helicopter in both hover and forward flight in the yaw, pitch, and roll axes.

Stabilization of a helicopter has been found to be desirable because of the basic instability of the uncontrolled craft. The stability augmentation system of this invention will compensate for the normal random attitude changes caused by atmospheric conditions. The pilot will thus find it unnecessary to continuously adjust the controls to keep the craft in the proper control attitude. The system will also provide short-term attitude hold capabilities for the craft in its commanded attitude.

One of the desirable features of this invention is the ability of the system to determine when the pilot is commanding a maneuver. When a specified input rate along one axis in one direction is maintained for a specified time interval, the attitude hold capabilities of the stability augmentation system will be disengaged and the craft will respond directly to the pilots control, with rate damping still being provided. However, the system will not fight the pilot when he wishes to change attitude.

The basic stability augmentation system incorporates a rate gyro to sense angular rates of the craft around the axis to be stabilized. The rate signal is then compensated to provide the required dynamic characteristics to give an output voltage proportional to the required correction signal necessary to stabilize the craft. This correction signal is amplified as necessary, and fed to a servo motor which drives a linear actuator. The linear actuator is mechanically or hydraulically coupled directly to the proper control surface on the craft. Position feedback proportional to the displacement of the linear actuator is provided by means of a feedback transducer.

A novel safety gate circuit is provided in the feedback loop to deactivate the entire control system in the event that a wire from the position feedback transducer opens or shorts to ground.

A novel lag-lead compensation circuit is incorporated in the system to provide the required correction signal to stabilize the craft and also to provide short-term attitude hold capabilities. A lag limiter circuit prevents an overdriving of the lag-lead compensation which might cause excessive delay in the return of the system to the symmetrical authority condition after a disturbance.

The system also incorporates a novel rate sensitive cornpensation change circuit which determines when the pilot is commanding a maneuver, and which shorts the lag portion of the series compensation leaving only rate feedback around the aircraft.

It is therefore an object of this invention to provide a novel stability augmentation system for an aircraft.

Another object of this invention is to provide a novel stability augmentation system for aircraft which provides short-term attitude hold capabilities for the craft.

A further object of this invention is a novel stability augmentation system for aircraft which will automatically return to a symmetrical authority system after a disturbance.

Another object of this invention is to provide a novel lag-lead compensation circuit for use in a stability augmentation system for aircraft.

3,199,013 Patented Aug. 3, 1965 A further object of this invention is a lag-lead compensation circuit in which a lag limiter prevents overdriving of the compensation.

Another object of this invention is a novel stability augmentation system for aircraft which will determine when the pilot is commanding a maneuver and automatically change the compensation of the system.

A further object of this invention is a novel rate sensitive compensation change circuit which eliminates system lag compensation when the craft rate of direction change is above a certain value in one direction for a given interval of time.

Another object of this invention is a safety gate circuit which deactivates the entire control system in the event that a wire from a position feedback transducer opens or shorts to ground.

Other objects and a fuller understanding of the invention may be had by referring to the following description and claims, read in conjunction with the accompanying drawings in which:

FIGURE l shows in block diagram form the stability augmentation system of this invention; and

FIGURE 2 is a schematic circuit diagram of the compensation networks of FIGURE l; and

FIGURE 3 is a schematic circuit diagram of the safety gate circuit of FIGURE l.

Referring now to FIGURE l, an aircraft, indicated as reference numeral 1t), may be disturbed from its controlled attitude by some outside source such as a wind gust. The angular rate of craft movement about the axis to be stabilized, for example the roll axis, is sensed by rate gyro 12. It should be pointed out here that the stability augmentation system may be used to stabilize the craft in either pitch, roll or yaw. Gyro 12 produces an A.C. output voltage which is proportional to the angular rate of craft deviation. The gyro output voltage is fed to the lag-lead compensation network 14 where the signal is compensated to provide the required dynamic characteristics to produce an output voltage proportional to the correction signal required to stabilize the craft.

After compensation, the rate signal is amplified as needed by amplier 16, and fed to servo motor 18. Motor 18 drives a linear actuator 2t) which provides a linear output velocity proportional to the applied rate-error signal. The linear actuator is connected through linkages to some control surface of the craft, such as an aileron or rudder. Pilot inputs are provided to the aircraft by the normal pilot controls.

Position feedback is provided by means of a linear voltage displacement transducer (LVDT) 22 which provides feedback proportional to the linear actuator displacement. A safety gate circuit 24 is provided to deactivate the control by opening the amplifier ground connection in the event that a wire in the position feedback transducer opens or shorts to ground. Safety gate circuit 24 will be described in detail later, and is shown schematically in FIGURE 3.

Lag limiter network 26 is connected with the lag-lead compensation network 14, and prevents an overdriving of the lag-lead compensation which might cause excessive delay in the return of the system to a symmetrical authority condition. The lag limiter will also be described in detail in connection with FIGURE 2.

Compensation change circuit 28 essentially responds to the rate input from gyro 12 and changes the lag-lead compensation of circuit 14 to full proportional compensation. A specified input rate in one direction must be maintained for a specified time interval for the compensation changer circuit to be activated. The circuit senses when the pilot is commanding a maneuver, and then destroys any attitude holding capabilities of the system and provides rate damping in the system. This function prevents the system from fighting the piiot when he wishes to change attitude. When a new attitude is achieved, and the input rate about the axis falls to zero, the system reverts back to the normal lag-lead compensation, and limited attitude hold capabilities are again provided at the commanded attitude. Since a specified input rate in one particular direction must be maintained for a specified time interval, random wind gust disturbances, which might cause high enough rates to trip the compensation changer circuit, will be discriminated against since they will not tend to be maintained in` one direction for a long enough interval of time. Therefore, the stabilizing effect of the system will be unimpaired.

FIGURE 2 shows schematically the electronics of laglead compensation circuit 14, lag limiter circuit 26 and compensation changeY circuit 2S. The A.C. input signals from rate gyro 12 is applied through resistor R1 across primary winding P1 of transformer T1. the rate gyro is a 40G-cycle A.C. signal having an amplitude proportional to the rate about the craft axis. The phase of the gyro output signal is dependent on the direction of deviation about the axis, the signal being either in phase or 180 out of phase with the gyro excitation signal.

The input signal across primary winding P1 is transferred to the secondaries of the transformer S1, S2 and S3. Winding S3 is the output winding, and this signal is fed directly to amplifier 16.

The signal appearing across secondary winding S1 and S2 is alternately chopped to ground by diode switches 30 and 32. A square-wave input supply having its center tap grounded supplies alternate positive and negative signals to terminals 34 and 34. The frequency of the square wave is identical to that of the gyro signal. The gyro excitation signal and the square wave signal are provided from the same inverter supply.

The square-wave input voltage is fed through lines 36 and 36 having series resistors 38 and 38 to opposite junctions of diode bridge 30. Likewise the square-wave voltage is fed through lines 40 and 40 having series resistors 42 and 42 to opposite junctions of diode bridge 32. The diodes in bridge circuits 30 and 32 are poled in such a direction that one of the bridge circuits is always conducting, and the bridge circuits conduct alternately as the square-wave input changes polarity. For example, when junction 34 is positive and 34 is negative, the diodes in bridge circuit 30 are forward biased. At the same time, the diodes in bridge circuit 32 are reverse biased. The diode bridge circuits act as switches, the switch being open when the diodes are not conducting and the switch being closed when the diodes are conducting. Thus with junction 34 positive and junction 34 negative, switch 30 is closed and switch 32 is opened. When the polarity of the square-wave signal changes so that junction 34 is negative and junction 34' is positive, switch 32 will be closed and switch 30 will be open.

As the input rate signal across P1 is transferred to Secondary windings S1 and S2, capacitor C1 will charge either positively or negatively depending on the phase relationship between the input signal and the squarewave switching supply. For example, when the input signal is such that the polarity of the signal at the end of the windings having the dot is positive with respect to the other end of the windings, and if diode bridge 30 is closed and diode bridge 32 is open at this time, diode bridge 30 will act as a substantially zero impedance to the signal. Thus the dotted portion of secondary winding S1 is effectively grounded. Capacitor C1 will charge negatively With respect to ground at this time. During the negative half cycle of the input signal, diode bridge 32 will be closed and diode bridge 30 will be open. However, since the polarity of the signal across secondary windings S1 and S2 has also changed, capacitors C1 will still receive a negative charge.

If the input rate to the gyro is in the opposite direction,

The output of the gyro output signal will be reversed in phase by and capacitor C1 will therefore be charged to a positive potential with respect to ground.

When an attitude change of the aircraft causes a step change in the envelope of the rate gyro output signal, capacitor C1 will act as an A.C. short and there will be no output across winding S3 at this time. The impedance of capacitor C1 will be reflected across the transformer to the primary circuit and appear across primary winding P1. Capacitor C1 will charge gradually and the output across secondary winding S3 will grow exponentially. The time constant of the circuit will be approximately If a step-up transformer is used, both C1 and R1 can be made small and still allow relatively long time constants in the circuit. There will be a very small voltage loss in the circuit if the inductance of Iprimary winding P1 is large.

In addition to the A.C. lag caused by the capacitor action as just described, an A.C. lead is also produced by the circuit. This is caused by the fact that the winding resistance of secondary windings S1 and S2 will alternately appear in series with C1. In addition, a small resistor may be placed in series with C1 to regulate the lead depending on the resistance of the secondary windings.

Thus the circuit as just described is a pure A.C. lag-lead circuit. When a step input appears across primary winding P1, the lead portion of the circuit produces an almost instantaneous small change in the signal across output winding S3. After this .instantaneous lead action, the output signal will build up exponentially because of the lag produced in the circuit. The equivalent circuit may be represented by a small resistor and a capacitor in series across primary winding P1.

The circuit lead may also be adjusted by adding proportional feedthrough by Way of resistors R2 and R3. Thus, in addition to laglead compensation applied to the rate signal, a pure rate signal is added to the system at winding S3. Varying the values of resistors R2 and R3 will adjust the amount of rate signal.

The linear actuator 20 is a mechanical device having stops which limit the authority of the position loop. Since the position loop is preceded by a lag produced in the series compensation, overdriving of the series compensation can ocur to produce an unwanted delay before the proportional loop is recentered. If the electrical output from the series compensation is limited after the mechanical limits have been reached, the overcharging and the resulting lag will be prevented. Lag limiter circuit 26 of FIGURE l limits the electrical output of the laglead compensation network. The lag limiting is provided by diodes CR1 and CR2 connected across capacitor C1. If the diodes are properly back biased by voltages of selected magnitudes, the diodes will conduct and limit the charge which can appear across capacitor C1.

Compensation change circuit 28 automatically varies the compensation of the system during maneuvers to eliminate the attitude holding capabilities of the system. If .it 1s assumed that a maneuver is being commanded by the pilot when a rate of change above a certain threshold value is maintained in one direction for a given interval of time, the output of the rate gyro may be used to determine when a maneuver is taking place. When the maneuver detector indicates lthat a maneuver is being commanded by the pilot, the lag portion of the series compensation is shorted, leaving only rate feedback around the aircraft.

FIGURE 2 shows the operation of the compensation change circuit. Referring now to FIGURE 2, the A.C. .rate signal from gyro 12 is fed across the primary winding of transformer T2. The secondary winding of trar1sformer T2 is connected across opposite junctions of diode bridge 44. A square-wave supply provides square-wave signals to junctions 46 and 46 which render the diodes of the `bridge circuit alternately conducting and non-conducting. Diode bridge 44 thus acts as a switch, and the entire circuit is a demodulator. The D.C. signal appearing at the center tap of the secondary Winding of transformer T2, which is proportional to the rate and direction of aircraft deviation about the axis, is fed to a lag network comprising resistor R1 and capacitor C2. The lagged D.C. signal is then applied through resistor R5 to the base junctions of transistors Q1 and Q2. Transistor Q1 is a p-n-p transistor, and is normally non-conducting. The emitter of transistor Q1 is normally biased somewhat negative from voltage source B- through resistors R8 and R6 to ground. Transistor Q2 is an n-p-ntransistor and is also normally non-conducting. The emitter junction of this transistor is normally biased somewhat positive by voltage source B-lthrough resistors R7 and R9 to ground. If the voltage across capacitor C2 sw-ings more negative than the voltage at the emitter of transistor Q1, Q1 will fire and provide a low impedance path for the rate signal from the gyro. This path will be through resistor R1, line 48, capacitor C5, through the transistor Q1 and through capacitor C3 to ground. This path will effectively short out the lag capacitor C1 by way of the transformer action of T1. Likewise if the voltage across capacitor C2 is more positive than the emitter voltage of transistor Q2, transistor Q2 will fire and the lag capacitor C1 will be shorted out through wire 48, capacitor C6, transistor Q2 and capacitor C1 to ground. The shorting action is caused primarily by capacitors C3, C1, C5 and C6 which provide the low A.C. impedance required when one or the other of the transistors fire. Thus regardless of whether the movement about the axis of the craft is clockwise or counterclockwise, the rate signal produced by the gyro, which yis proportional to both rate and direction, will be fed across lag capacitor C2. If the signal lasts for a time determined by the time constant of the lag circuit, the lag capacitor C2 will be charged sufficiently to switch on either transistor Q1 or Q2 and short the rate gyro input signal around the laglead compensation network. Thus the circuit senses when the pilot has deliberately commanded the craft attitude change, and eliminates this compensation portion of the stability augmentation system. The only signal which now `appears is the pure rate feedback around the aircraft via resistors R2 and R3.

FIGURE 3 shows schematically the linear voltage displacement transducer 22 and the safety gate circuit 24. Slug S0 is directly connected to the output of the linear actuator 20, and is moved back and forth between the primary and secondary windings of transformer T3 to vary the coupling across the transformer. An alternating current supply, not shown, provides A.C. across the primary winding of transformer T3. The voltage across secondary winding S, which is determined by the position of the slug Sti, is fed to the input of amplifier 16. This feedback signal is proportional to the displacement of the linear actuator.

The safety gate circuit comprises a DC. path around the transformer T3 which will sense if either the primary or secondary windings are opened or shorted to ground. A source of D.C. voltage, B+, provides current through resistor R10, through primary winding P, then through resistor R11 and through secondary winding S to ground through resistor R12. Capacitors C7 and C8 block the D.C. voltage in the A.C. transformer lines. The amount of current provided through the path need only be suticient to cause a large enough voltage drop across resistor R12 to produce a positive bias at the base of transistor Q3 to keep transistor Q3 saturated. Transistor Q3 is part of the ground system for the first stage of amplifier 16. If either primary winding T or secondary winding S of transformer T3 opens or shorts to ground, the current path will be broken and transistor Q3 will be turned olf because there will no longer be a base voltage. This will open the ground system for the power amplifier, and the entire system will be activated. Capacitor C9 across resistor R12 acts as an A.C. ground return path.

While the invention has been described in terms of a stability augmentation system for aircraft, and particularly for helicopters, it need not be limited thereto but may be used wherever stability augmentation or automatic stabilization may be desirable. One obvious example is the use of such a system for stabilizing hydrofoil craft. In addition, the novel circuits described need not be limited to their specific function in this system, but may be used wherever an electronic circuit of this sort is desirable.

While the invention has been described in its preferred embodiment, it need not be limited thereto. Many modifications and changes may be made in the details of construction without departing from the scope of the invention as hereinafter claimed.

We claim:

1. A stability augmentation system for a craft comprising means for producing an error signal proportional to the rate of craft motion about an axis, circuit means compensating said error signal to produce a correction signal, a control surface on said craft, servo means including an actuator responsive to said correction signal and to said error signal for actuating said control surface to thereby counteract said craft motion, and means responsive to a predetermined rate of change of craft motion about said axis in one direction for a specified minimum time interval for disabling said circuit means and eliminating said correction signal.

2. A stability augmentation system for a craft comprising gyro means responsive to rate of craft motion about an axis for producing a proportional error signal, circuit means compensating said error signal to produce a correction signal, a control surface on said craft, servo means including a linear actuator, means connecting said correction signal to said servo means, additional means connecting said error signal to said servo means, said servo means being responsive to said correction signal and to said proportional error signal for actuating said control surfaceto thereby counteract said craft motion, and additional circuit means responsive to a predetermined rate of change of craft motion about said axis in one direction for a specified time interval for disabling said circuit means and eiiminating said correction signal.

3. A stability augmentation system as in claim 2 in which `said circuit means is reactivated when said error signal falls to substantially zero.

4. A stability augmentation system as in claim 2 in which said circuit means comprises a lag-lead compensation circuit.

5. A stability augmentation system for a craft comprising means for producing an error signal proportional to the rate of craft motion about an axis, circuit means compensating said error `signal to produce a correction signal, control surface means on said craft, servo means responsive to said error signal and to said correction signal for actuating said control surface means to thereby counteract said craft motion, switching means connected with said circuit means, and means connecting said error signal with said switching means, said switching means being actuated when said error signal remains at a preselected value for a specified time interval to thereby disable said circuit means and eliminate said correction signal.

6. A stability augmentation system as in claim 5 in which said means connecting said error signal with said switching means includes a capacitor, said capacitor being charged by said error signal to a voltage sufficient to actuate said switching means when said error signal remains at a preselected value for a specified time interval.

7. A stability augmentation system as in claim 6 in which said switching means includes first and second transistors, and means for biasing said transistors to their non-conducting state, a charge on said capacitor above a predetermined magnitude of one polarity rendering said first transistor conductive, a charge on said capacitor above a predetermined magnitude of the other polarity rendering said second transistor conductive.

8. In a stability augmentation system for a craft, a rate gyro responsive to craft rate about an axis for producing a craft rate signal, means including a compensation circuit for converting said rate signal into a craft correction signal, control surface means on said craft, servo means responsive to said craft rate signal and said craft correction signal for actuating said control surface means to counteract said craft motion, a lag circuit including a capacitor, means connecting said rate signal with said lag circuit to charge said capacitor, a switching circuit connected between said compensation circuit and ground, and means connecting said capacitor with said switching circuit, said switching circuit being actuated by a charge on said capacitor above a preselected magnitude to conduct said rate signal to ground and deactivate said compensation circuit.

9. A stability augmentation for a craft comprising means for producing an error signal proportional to the rate of craft motion about an axis, control surface means on said craft, servo means for actuating said control surface means to eliminate said error signal, a rst circuit path conducting said error signal to said servo means, a second circuit path including a lag-lead circuit for cornpensating said error signal and feeding said compensated error signal to said servo means, and means responsive to craft motions produced in response to pilot commands for deactivating said second circuit path.

iii. A stability augmentation system asin claim 9 and including an auxiliary circuit path for said error signal by which said error signal is connected through a switching circuit to ground, means for normally biasing said switching circuit into its nonconducting state, a capacitor connected to said biasing means, and a second auxiliary circuit path connecting said error signal to said capacitor, said capacitor being charged by an error signal produced in response to pilot commands to override said biasing means and render said switching circuit conductive. 

1. A STABILITY AUGMENTATION SYSTEM FOR A CRAFT COMPRISING MEANS FOR PRODUCING AN ERROR SIGNAL PROPORTIONAL TO THE RATE OF CRAFT MOTION ABOUT AN AXIS, CIRCUIT MEANS COMPENSATING SAID ERROR SIGNAL TO PRODUCE A CORRECTION SIGNAL, A CONTROL SURFACE ON SAID CRAFT, SERVO MEANS INCLUDING AN ACTUATOR RESPONSIVE TO SAID CORRECTION SIGNAL AND TO SAID ERROR SIGNAL FOR ACUTATING SAID CONTROL SURFACE TO THEREBY COUNTERACT SAID CRAFT MOTION, AND MEANS RESPONSIVE TO A PREDETERMINED RATE OF CHANGE OF CARFT MOTION ABOUT SAID AXIS IN ONE DIRECTION FOR A SPECIFIED MINIMUM TIME INTERVAL FOR DISABLING SAID CIRCUIT MEANS AND ELIMINATING SAID CORRECTION SIGNAL. 